Induction motors frequently are used in industrial applications. A common type of induction motor (also known as an asynchronous motor) utilizes multi-phase, typically three-phase, alternating current (AC) to energize the stator coils. In industrial applications, such AC induction motors can be quite large, driven by high current to develop high torque output. It is often necessary to operate such motors in high-temperature, corrosive environments.
One common application for such large induction motors is in diesel-electric locomotives. In such an environment, AC induction motors may be controlled by the master computer that controls the electrical systems for the locomotive. The master computer typically sends operation signals to a control board having a controller that, in turn, operates switches to open and close connections between the motor and a source of three-phase electrical power produced by a diesel-electric AC generator.
In one application, which may be to drive an exhaust or cooling fan for the diesel engine enclosure, the AC induction motor is configured for either high-speed or low-speed operation. In high-speed operation, coils in a wye configuration are connected to a source of three-phase power, and in a low-speed configuration the coils connected in a delta configuration are similarly energized.
When operating such motors that ultimately are controlled by a master computer, it is desirable to avoid a situation in which shorting across coils may occur, as this may damage or burn up the motor. Conditions in which switches may become overheated may occur, which also may damage the system and motor. Further, as a result of operational or programming errors, the master computer may transmit signals to the motor controller for high-speed and low-speed operation simultaneously, which if implemented by the controller would damage the motor and the control system.
Accordingly, there is a need for a control system for an induction motor that is sufficiently robust to operate in harsh environments, and that minimizes the likelihood of motor damage as a result of shorting, high temperature or incorrect operation signals.